The present invention relates to a method of driving a solid-state imaging element such as a CCD and, more particularly, to a method of controlling the exposure amount of a solid state imaging device using a solid-state imaging element.
Conventional methods of controlling the exposure amount of not only solid-state imaging devices but also other imaging devices have been realized by a combination of control of the aperture of a lens and of a shutter speed. In a video camera, its shutter speed is determined by the field frequency of the television format. For example, in the NTSC format, the shutter speed is 1/60 seconds. Therefore, the aperture is adjusted in accordance with changes in incident light amount so as to optimize the exposure amount. In addition, in order to adjust the exposure amount of a device, other techniques are employed, e.g., changing the gain of a camera or inserting an ND filter in front of a lens. As solid state imaging elements have been increasingly used in practical applications, various driving methods have been developed. Especially, an electronic shutter is designed to increase the dynamic resolution by discharging some of charges generated by photoelectric conversion to the drain. With this shutter, exposure amount control is also realized.
FIG. 6 shows a structure of a device which realizes an electronic shutter. FIG. 7 shows changes in the amount of charges accumulated in a photoelectric conversion element 1 as a function of time. Incident light is photoelectrically converted by the photoelectric conversion element, and the resultant charges are accumulated. More specifically, if light is incident with the same intensity for an interval of one field, the amount of charges is linearly increased from t.sub.0 to t.sub.2, as shown in FIG. 7. If, however, a pulse is applied to an overflow control gate electrode at a given timing t.sub.1 so as to discharge the charges of the photoelectric conversion element 1 to an overflow drain 7 and to clear them, an output at a timing when the charges are transferred to a vertical transfer register 2, i.e., at the timing t.sub.2, can be set to be (t.sub.2 -t.sub.1)/(t.sub.2 -t.sub.0) of one normal operation.
That is, by equivalently setting the interval between t.sub.1 and t.sub.2 as a charge storage time, a shutter operation capable of increasing the dynamic resolution can be realized, and adjustment of an exposure amount can also be performed.
In the method of controlling the exposure amount by this shutter operation, the dynamic resolution is increased. However, since a picture is intermittently generated every 1/60 seconds due to the limitations of a television format, especially when a high shutter speed is set, the shutter operation becomes discontinuous, involving a sense of awkwardness.